1. Field of the Invention
The present invention relates to a magnetic bearing device and a turbo molecular pump with the magnetic bearing device mounted thereto. More specifically, the present invention relates to a magnetic bearing device capable of reducing vibration by preventing a deterioration in the control of a rotor shaft due to a variation in the axial position of the rotor shaft, while performing positional control over the rotor shaft in a specific posture with optimal magnetic force, and to a turbo molecular pump with the magnetic bearing device mounted thereto.
2. Description of the Related Art
With the development of electronics in recent years, demands for semiconductors for forming memories, integrated circuits, etc. are rapidly increasing. Those semiconductors are manufactured such that impurities are doped into a semiconductor substrate with a very high purity to impart electrical properties thereto, or semiconductor substrates with minute circuit patterns formed thereon are laminated. Those manufacturing steps must be performed in a chamber with a high vacuum state so as to avoid influences of dust etc. in the air. As a pump device, a vacuum pump is generally used to evacuate this chamber. In particular, a turbo molecular pump, one kind of the vacuum pump, is widely used since it entails little residual gas and is easy of maintenance. The semiconductor manufacturing process includes a number of steps in which various process gases are caused to act onto a semiconductor substrate, and the turbo molecular pump is used not only to evacuate the chamber but also to discharge those process gases from the chamber.
Further, in equipment for an electron microscope etc., a turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope etc. in order to prevent refraction etc. of an electron beam caused by the presence of dust or the like. FIG. 10 is a vertical sectional view of this turbo molecular pump.
In FIG. 10, a turbo molecular pump 100 includes an outer cylinder 127 with an intake hole 101 formed on top thereof. Provided inside the outer cylinder 127 is a rotor 103 having in its periphery a plurality of rotor blades 102a, 102b, 102c . . . serving as turbine blades for sucking and discharging gas and formed radially in a number of stages. At the center of the rotor 103, a rotor shaft 113 is mounted with being supported in a levitating state in the air and controlled in position, for example, by a 5-axis control magnetic bearing.
Upper radial electromagnet 104 includes four electromagnets arranged in pairs in X- and Y-axis and plus- and minus-side directions (although not shown in the drawing, one group of electromagnets on the plus-side is referred to as upper radial electromagnet 104+, while the other group of electromagnets on the minus-side is referred to as upper radial electromagnet 104−. Further, there is provided an upper radial sensor 107 constituted of four electromagnets arranged in close proximity to and in correspondence with the upper radial electromagnets 104.
The upper radial sensor 107 is an inductance type sensor of a differential type by which the position of the rotor shaft 113 in X-axis direction and that in Y-axis direction are each detected from the two directions of plus- and minus-side directions, and by which a sensor signal corresponding to the radial position of the rotor shaft 113 is output to a magnetic bearing control section of a control device or the like (not shown in the drawing). The magnetic bearing control section excites and controls the upper radial electromagnets 104 based on the sensor signal from the upper radial sensor 107, thus the radial position of the upper portion of the rotor shaft 113 being adjusted.
The rotor shaft 113 is formed of a high-magnetic-permeability material (e.g., iron) and is adapted to be attracted by the magnetic force of the upper radial electromagnets 104. Such adjustment is conducted independently in the X-axis direction and the Y-axis direction.
Similarly, lower radial electromagnet 105 includes four electromagnets arranged in pairs in X- and Y-axis and plus- and minus-side directions (although not shown in the drawing, one group of electromagnets on the plus-side is referred to as lower radial electromagnet 105+, while the other group of electromagnets on the minus-side is referred to as lower radial electromagnet 105−. Further, there is provided a lower radial sensor 108 constituted of four electromagnets arranged in close proximity to and in correspondence with the lower radial electromagnets 105.
This lower radial sensor 108 is also an inductance type sensor of a differential type. The radial position of the lower portion of the rotor shaft 113 is adjusted by a magnetic bearing control section such as a control device (not shown in the drawing).
Further, axial electromagnets 106 are arranged on the upper and lower sides of a metal disc 111 provided in the lower portion of the rotor shaft 113 (in FIG. 10, one electromagnet on the upper side is referred to as an axial electromagnet 106− on the minus-side, while the other electromagnet on the lower side is referred to as an axial electromagnet 106+ on the plus side). The axial electromagnet 106− on the minus-side attracts, by the magnetic force, the magnetic disc 111 toward the intake hole 101 side, and the axial electromagnet 106+ on the plus side attracts the magnetic disc 111 toward a base portion 129 side. The metal disc 111 is formed of a high-magnetic-permeability material such as iron.
In the base portion 129 arranged at the bottom of the outer cylinder 127, an axial sensor 109 is provided to detect an axial position Z of the rotor shaft 113. This axial sensor 109 detects the axial position Z of the rotor shaft 113 by detecting the position of a sensor target 110 embedded in the lower end portion of the rotor shaft 113, and outputs a sensor signal corresponding to this axial position Z to a control device such as a magnetic bearing control section 150 (will be described later with reference to FIG. 11).
The axial sensor 109 is an inductance type sensor as well as the upper radial sensor 107 and the lower radial sensor 108, and is provided only at the lower end side of the rotor shaft 113 to detect the axial position Z of the rotor shaft 113 from only one direction. Because installing another axial sensor at the upper side, which is difficult in itself, causes the complexity in structure. This inductance type sensor provides a sufficient quality in vibration with the turbo molecular pump 100, which is used with a great emphasis on its flow rate. Thus, in the turbo molecular pump 100 in existence, the axial sensor 109 is not a differential type sensor to avoid an increase in the number of components lead to an increase in cost. Therefore, the axial sensor 109 is different from the above-described upper radial sensor 107 and lower radial sensor 108 in that it is an inductance type sensor of a single-acting type.
The axial electromagnets 106 are excited and controlled by the magnetic bearing control section 150 on the basis of the sensor signal from the axial sensor 109. In this way, the magnetic bearing control section 150 has a function to appropriately control the magnetic force exerted on the metal disc 111 by the axial electromagnets 106 to magnetically levitate the rotor shaft 113 in the axial direction, thereby retaining the rotor shaft 113 in the space in a non-contact state.
A motor 121 is equipped with a plurality of magnetic poles, which are arranged circumferentially to surround the rotor shaft 113. The magnetic poles are controlled by a control device to rotate the rotor shaft 113 through an electromagnetic force acting between the rotor shaft 113 and the magnetic poles. The motor 121 also has an RPM sensor (not shown in the drawing) incorporated to output a detection signal, which is used for detection of RPM of the rotor shaft 113. A phase sensor (not shown in the drawing) is attached in the vicinity of the lower radial sensor 108 to detect the phase of rotation of the rotor shaft 113.
A plurality of stationary blades 123a, 123b, 123c . . . are arranged so as to be spaced apart from the rotor blades 102a, 102b, 102c . . . by small gaps. To downwardly transfer the molecules of exhaust gas through collision, the rotor blades 102a, 102b, 102c . . . are inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113. Similarly, the stationary blades 123 are also inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113, and extend toward the inner side of the outer cylinder 127 to be arranged alternately with the rotor blades 102.
The stationary blades 123 are supported at one end by being inserted into gaps between a plurality of stationary blade spacers 125a, 125b, 125c . . . stacked together in stages. The stationary blade spacers 125 are ring-shaped members, which are formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metal as a component.
In the outer periphery of the stationary blade spacers 125, the outer cylinder 127 is secured in position with a small gap therebetween. At the bottom of the outer cylinder 127, there is arranged a base portion 129, and a threaded spacer 131 is arranged between the lowermost one of the stationary blade spacers 125 and the base portion 129. In the portion of the base portion 129 below the threaded spacer 131, there is formed a discharge outlet 133 which communicates with the outside. The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metal as a component, and has a plurality of spiral thread grooves 131a in its inner peripheral surface. The spiral direction of the thread grooves 131a is determined such that when the molecules of the exhaust gas move in the rotating direction of the rotor 103, these molecules are transferred toward the discharge outlet 133.
Connected to the lowermost one of the rotor blades 102a, 102b, 102c . . . of the rotor 103 is a rotor blade 102d, which extends vertically downwards. The outer peripheral surface of the rotor blade 102d sticks out toward the inner peripheral surface of the threaded spacer 131 in a cylindrical shape, and is in close proximity to the inner peripheral surface of the threaded spacer 131 with a predetermined gap therebetween.
The base portion 129 is a disc-like member constituting the base of the turbo molecular pump 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel. The base portion 129 physically retains the turbo molecular pump 100, and also functions as a heat conduction passage. Thus, the base portion 129 is preferably formed of a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper.
In the above-described construction, when the rotor blades 102 are driven and rotated by the motor 121 together with the rotor shaft 113, an exhaust gas from a chamber is sucked in through the intake hole 101 by the action of the rotor blades 102 and the stationary blades 123. The exhaust gas sucked in through the intake hole 101 passes between the rotor blades 102 and the stationary blades 123, and is transferred to the base portion 129. At this point, the temperature of the rotor blades 102 is raised by frictional heat generated as the exhaust gas comes into contact with the rotor blades 102 and by heat generated and conducted from the motor 121. Such heat is transferred to the stationary blades 123 through radiation or through conduction of gas molecules of exhaust gas or the like.
The stationary blade spacers 125 are joined to one another on the outer periphery and send, to the outside, heat which the stationary blades 123 receive from the rotor blades 102 as well as frictional heat generated upon contact between exhaust gas and the stationary blades 123. The exhaust gas transferred to the base portion 129 is sent to the discharge outlet 133 while being guided by the thread grooves 131a of the threaded spacer 131.
The turbo molecular pump 100 requires control based on individually adjusted specific parameters (e.g., identification of the model and characteristics corresponding to the model). To store the control parameters, the turbo molecular pump main body 100 contains an electronic circuit portion 141 in its main body. The electronic circuit portion 141 is composed of a semiconductor memory, such as EEP-ROM, electronic parts, such as semiconductor devices for access to the semiconductor memory, a substrate 143 for mounting these components thereto, etc. This electronic circuit portion 141 is accommodated at the portion below the center of the base portion 129, and is closed by a hermetic bottom cover 145.
Given next is a detailed description of the magnetic bearing control section 150 for exciting and controlling the axial electromagnets 106 of the turbo molecular pump 100 structured as above.
A block diagram of a conventional magnetic bearing control section 150 is shown in FIG. 11. Although the following description is about the magnetic bearing control section 150 for controlling the axial electromagnets 106, other magnetic bearing control sections for controlling the upper radial electromagnets 104 or the lower radial electromagnets 105 have the same structure if not otherwise specified.
In FIG. 11, the sensor signal amplifier section 152 of the magnetic bearing control section 150 is input with a sensor signal output from the axial sensor 109. This sensor signal is a signal corresponding to the axial position Z of the rotor shaft 113. The sensor signal amplifier section 152 performs a fixed sensor sensitivity adjustment to the sensor signal to output an sensor output signal 153 after the sensitivity adjustment to an A/D converter 154. The sensor sensitivity adjustment means adjusting a gain value or the like of the sensor signal amplifier section 152. The sensor output signal 153 output from the sensor signal amplifier section 152 has a predetermined voltage value (hereinafter referred to as a sensor output voltage V) by this sensor sensitivity adjustment.
The A/D converter 154 performs digital conversion to the sensor output voltage V of the sensor output signal 153 to output a converted digital sensor signal 155 to a levitation control section 156. The levitation control section 156 performs a PID adjustment based on the digital sensor signal 155 or the like to output an electromagnet excitation signal 157 to an electromagnet excitation section 158. The electromagnet excitation section 158 is constituted, for example, of a bridge circuit having two transistors and two diodes, and increases or reduces, depending on the electromagnet excitation signal 157, the electromagnetic current caused to flow between the electromagnet excitation section 158 and the axial electromagnets 106.
At this point, the levitation control section 156 is input with a levitation constant signal 162, which represents a levitation constant F required for the control or the like of the electromagnets 104, 105, and 106. This levitation constant F is previously set in a levitation constant setting section 161 and includes constants for adjusting the magnetic force caused in the axial electromagnets 106, such as a constant for determining a value P of the PID adjustment in the levitation control section 156, and a constant for determining a stationary current value of the electromagnet current caused to flow in the axial electromagnets 106 or the like via the electromagnet excitation section 158.
In this structure, when the axial position Z of the rotor shaft 113 changes, the sensor signal input to the magnetic bearing control section 150 for controlling the axial electromagnets 106 is changed, thereby the sensor output voltage V of the sensor output signal 153 is changed. The relation between this axial position Z and the sensor output voltage V is shown in FIG. 12.
In FIG. 12, the horizontal axis represents the axial position Z while the vertical axis represents the sensor output voltage V. At this point, differently from the upper radial sensor 107 and the lower radial sensor 108, the axial sensor 109 is an inductance type sensor of a single-acting type in consideration of an increase in the number of components or the like. The sensor output voltage V is in inverse proportion to the square of the axial position Z (V∞1/ZZ). Thus, the relation between the axial position Z and the sensor output voltage V can be represented as a curved line 196. In the case of the upper radial sensor 107 and the lower radial sensor 108, each of which is an inductance type sensor of a differential type, the relation between the axial position Z and the sensor output voltage V can be represented as a linear curve (not shown in the drawing).
Then, the sensor output voltage V has an upper limit value 197 and a lower limit value 198 that are determined by the characteristic of the A/D converter 154, to which the sensor output signal 153 is input. Thus, the axial position Z, corresponding to the sensor output voltage V, has a minimum value Zmin with respect to the upper limit value 197, and a maximum value Zmax with respect to the lower limit value 198. Specifically, when the axial position Z gets below the minimum value Zmin or above the maximum value Zmax, the change in the axial position Z cannot be detected by the A/D converter 154. It is noted that such a turbo molecular pump 100 as in which the axial position Z is not within the range from the minimum value Zmin to the maximum value Zmax, due to manufacturing process or the like, is subject to be discarded or reassembled, for example.
As shown in FIG. 12, when the axial position Z is moved further from the axial sensor 109 by a displacement ΔZ for example, then the sensor output voltage V is lowered by a voltage change amount ΔV in accordance with this displacement ΔZ. When this voltage change amount ΔV is detected by the A/D converter 154, the digital sensor signal 155 is changed. Thus, the position of the rotor shaft 113 can be adjusted by letting the levitation control section 156 and the electromagnet excitation section 158 to excite and control the axial electromagnets 106 based on the change of the digital sensor signal 155. The same applies to the case where the axial position Z is moved closer to the axial sensor 109.
By the way, the axial position Z of the rotor shaft 113 is generally set by the μm with respect to the axial sensor 109. Thus, in some cases, there could be a variation, due to manufacturing process, in the axial position Z of the rotor shaft 113 among a plurality of turbo molecular pumps 100, 100 . . . .
This variation in the axial position is shown in FIG. 13. Among a plurality of turbo molecular pumps 100, 100 . . . in each of which the axial position Z is within the range from the minimum value Zmin to the maximum value Zmax as described above: the turbo molecular pump 100 in FIG. 13(a) (hereinafter referred to as a turbo molecular pump 100A) has the narrowest gap between the axial sensor 109 and the sensor target 110; and the turbo molecular pump 100 in FIG. 13(b) (hereinafter referred to as a turbo molecular pump 100B) has the widest gap between the axial sensor 109 and the sensor target 110. The relation between the axial position Z and the sensor output voltage V for these two types of turbo molecular pumps 100A and 100B is shown in FIG. 14.
In FIG. 13 and FIG. 14, in the case of the turbo molecular pump 100A having a narrow gap in the axial direction, a proximal position Za1, at which the rotor shaft 113 is proximal to the axial sensor 109, substantially coincides with the minimum value Zmin of the axial position Z, which is determined by the characteristic of the A/D converter 154, with a margin Zt1 therebetween. In this turbo molecular pump 100A, the rotor shaft 113 can move within the range of a movable stroke La, which is from the proximal position Za1 to a distal position Za2. As a result, the sensor output voltage V can be changed by a voltage change amount Δva.
At this point, each of the proximal position Za1 and the distal position Za2 of the rotor shaft 113 is a position that is determined by the arrangement of the rotor shaft 113, the metal disc 111, and a bearing 120 or the like. Thus, as shown in FIG. 10, when the rotor shaft 113 is at the proximal position Za1, a gap 137 between the rotor shaft 113 and the bearing 120 is cleared, while when the rotor shaft 113 is at the distal position Za2, a gap 138 between the bearing 120 and the metal disc 111 is cleared.
On the other hand, in the case of the turbo molecular pump 100B having a wide gap in the axial direction, a distal position Zb2, at which the rotor shaft 113 is distal to the axial sensor 109, substantially coincides with the maximum value Zmax of the axial position Z, which is determined by the characteristic of the A/D converter 154, with a margin Zt2 therebetween. In this turbo molecular pump 100B, the rotor shaft 113 can move within the range of a movable stroke Lb, which is from the distal position Zb2 to a proximal position Zb1. As a result, the sensor output voltage V can be changed by a voltage change amount ΔVb. Each of the proximal position Zb1 and the distal position Zb2 of the rotor shaft 113 is also determined by the arrangement of the rotor shaft 113, the metal disc 111, and the bearing 120 or the like.
In some cases, there could be a variation, due to the arrangement of the rotor shaft 113, the metal disc 111, and the bearing 120 or the like, in the movable strokes La and Lb among a plurality of turbo molecular pumps 100, 100 . . . However, in FIG. 14, it is assumed that La equals Lb for simplification.
By the way, in the turbo molecular pump 100A having a narrow gap in the axial direction, when a voltage change amount ΔVa of the sensor output voltage V per the movable stroke La is defined as a resolution Ra, the definition can be expressed as Ra=ΔVa/La. On the other hand, in the turbo molecular pump 100B having a wide gap in the axial direction, when a voltage change amount ΔVb of the sensor output voltage V per the movable stroke Lb is defined as a resolution Rb, the definition can be expressed as Rb=ΔVb/Lb The resolutions Ra and Rb represent derivative value dV/dZ of the sensor output voltage V in the strokes La and Lb respectively.
At this point, when the axial position Z of the rotor shaft 113 is within the range from the minimum value Zmin to the maximum value Zmax, the sensor output voltage V is in inverse proportion to the square of the axial position Z of the rotor shaft 113 (V∞1/ZZ). Thus, the closer the axial position Z of the rotor shaft 113 approaches to the minimum value Zmin, the larger the derivative value dV/dZ of the sensor output voltage V becomes. While, the further the axial position Z the rotor shaft 113 recedes from the minimum value Zmin, the smaller the derivative value dV/dZ of the sensor output voltage V becomes.
Thus, the resolution Rb in the turbo molecular pump 100B having a wide gap in the axial direction could become lower than the resolution Ra in the turbo molecular pump 100A having a narrow gap in the axial direction. Due to this reason, even when the displacement ΔZ generated in the rotor shaft 113 of each turbo molecular pump is the same, the voltage change amount ΔVb of the turbo molecular pump 100B becomes smaller than the voltage change amount ΔVa of the turbo molecular pump 100A, by which a minute displacement ΔZ of the rotor shaft 113 in the turbo molecular pump 100B is difficult to be detected, resulting in a deterioration in the control of the rotor shaft 113.
Thus, the rotor shaft 113 of the turbo molecular pump 100B having a wide gap could vibrate harder than that of the turbo molecular pump 100A having a narrow gap, which makes the entire turbo molecular pump 100B vibrated, thereby the operation in the chamber is affected. Further, a hard vibration of the rotor 103 could increase heat generated from the rotor 103 itself and the axial electromagnets 106 leading to wasteful energy consumption, and breaks the rotor blades 102 or the like.
Furthermore, even when the vibration of the turbo molecular pump 100A having a narrow gap can be further reduced by, for example, adjusting the magnetic bearing control section 150, the adjustment or the like of the magnetic bearing control section 150 has been performed to satisfy both of the turbo molecular pump 100A, and the turbo molecular pump 100B having a wide gap in the axial direction with less control. As a result, the adjustment cannot be said to be optimal for the turbo molecular pump 100A.
To reduce a variation in the axial position of the rotor shaft 113, the turbo molecular pump 100B or the like may be separately provided with such a mechanism as to automatically adjust the axial direction of the rotor shaft 113 (not shown in the drawing). However, such a mechanism, for which a high accuracy and complexity are required, has caused a possibility of the increase in cost for components. Further, such a mechanism, for which an additional step for adjusting the axial position of the rotor shaft 113 in manufacturing process is required, has also caused a possibility of the increase in cost for manufacturing.
In addition, generally, the turbo molecular pump 100 should be designed so that it can operate in any setting posture, such as a vertical setting posture (with the intake hole 101 directed in the counter-gravitational direction), an upside-down setting posture (with the intake hole 101 directed in the gravitational direction), and a horizontal setting posture (with the intake hole 101 directed vertical with respect in the gravitational direction).
When the turbo molecular pump 100 is installed in the vertical posture, the electromagnet that requires the highest magnetic force among the upper radial electromagnets 104+ and 104−, the lower radial electromagnets 105+ and 105−, and the axial electromagnets 106+ and 106− is the axial electromagnet 106−, which resists a predetermined disturbance against the rotor shaft 113 while attracting the rotor shaft 113 in the counter-gravitational direction. On the other hand, each of the other electromagnets 104+, 104−, 105+, 105−, and 106+ requires a magnetic force enough only to resist the disturbance, and thus doesn't require such a strong magnetic force as required by the axial electromagnet 106−.
Similarly, when the turbo molecular pump 100 is installed in the upside-down posture, the axial electromagnet 106+, which attracts the rotor shaft 113 in the counter-gravitational direction, requires a strong magnetic force, while each of the other electromagnets 104+, 104−, 105+, 105−, and 106− doesn't require a strong magnetic force. Similarly still, when the turbo molecular pump 100 is installed in the horizontal posture, each of the radial electromagnets 104+, 104−, 105+, and 105− require a strong magnetic force, while each of the axial electromagnets 106+ and 106− doesn't require a strong magnetic force.
As described above, depending on the setting posture of the turbo molecular pump 100, each of the electromagnets 104+, 104−, 105+, 105−, 106+, and 106− can be classified as an electromagnet that requires a strong magnetic force, or an electromagnet that does not require a strong magnetic force.
However, in the conventional turbo molecular pump 100, the levitation constant F, by which a strong magnetic force can be generated in each of the electromagnets 104+, 104−, 105+, 105−, 106+, and 106−, has been set (in the levitation constant setting section 161 of FIG. 11) to enables the rotor shaft 113 to be levitated in any setting posture.
As a result, even when the setting posture of the turbo molecular pump 100 is specified (for example, when it is clear that the turbo molecular pump 100 be installed in the vertical posture), a magnetic force stronger then requires could be generated in the electromagnet that does not attract the rotor shaft 113 in the counter-gravitational direction (e.g., electromagnets 104+, 104−, 105+, 105−, and 106+), causing in the vibration of the rotor shaft 113. Thus, the vibration of the entire turbo molecular pump 100B could affect the operation in the chamber. Further, a hard vibration of the rotor 103 could increase heat generated from the rotor 103 itself and the axial electromagnets 106 leading to wasteful energy consumption, and breaks the rotor blades 102 or the like.